1. Technical Field
This disclosure relates to a thermal recording adhesive label, and more particularly to a thermal recording adhesive label having sufficient adhesiveness with a little adhesive and no deterioration of colorability of its thermal recording layer after stored.
2. Discussion of the Related Art
Recently, a variety of recording materials are researched, developed and put into practical use with diversification and expanded needs of information. Among the recording materials, thermal recording materials have advantages of (1) being capable of simply recording images with only heating processes and (2) being capable of making apparatuses required compact because its mechanism is simple, and being easy to handle with and inexpensive. Therefore, the thermal recording materials are widely used in an information processing field such as output from desk calculators, computers, etc.; a recorder field such as medical measurements; low to high-speed facsimile fields; an automatic ticket vendor field such as railway tickets and entrance tickets; a thermal copying field; a label for POS system field; a tag field, etc.
The thermal recording adhesive label typically has a structure in which a substrate, a thermal recording layer, an adhesive layer and a release paper are layered. The release paper includes high-density base papers such as glassine papers, clay-coated papers, and polyethylene laminated papers coated with release agents such as silicone compounds and fluorine-containing compounds. Adhesives used is the adhesive layer include emulsion types such as rubbery, acrylic, vinylether adhesives; solvent types; hot melt adhesives, etc. Particularly, the acrylic emulsion type adhesives are widely used in terms of safety, quality and cost.
However, the emulsion type adhesives are likely to be inferior to solvent type adhesives in adhesiveness, cohesion and tackiness. Therefore, the emulsion type adhesives need to be coated more than the solvent type adhesives when preparing high-quality adhesive labels.
In addition, the thermal recording adhesive labels are typically required to have preservation stability of images recorded thereon. Therefore, methods of coating an aqueous emulsion of a resin having film formability and chemical resistance and methods of coating water-soluble polymeric compounds such as polyvinylalcohol are disclosed. The adhesive layer and release paper are formed on the back side of the substrate, and the above-mentioned rubbery and acrylic adhesives are typically used in the adhesive layer. Particularly, acrylic emulsion type adhesives are mostly used. Therefore, it is known that even before used as labels, low-molecular-weight oligomers, surfactants, etc. included in the adhesive layer enter the thermal recording layer, resulting in deterioration of the recording sensitivity thereof and occasional white spots of images in an extreme case.
Japanese published unexamined application No. 8-100156 discloses forming an undercoat layer (back coat layer) including a polypropylene resin or esteracrylate modified with an alkyleneimine compound as a main component on the back side of the substrate to solve this problem. However, this can prevent the adhesive from entering the thermal recording layer, but has insufficient adhesiveness unless not a little amount of the adhesive is used. Japanese published unexamined application No. 2006-143948 discloses a film including layered adhesives made from amorphous α-olefin copolymers. However, this has insufficient adhesiveness and does not work as an adhesive label. Further, Japanese published unexamined applications Nos. 2007-076252 and 2007-204549 disclose a thermal adhesive material in which an undercoat layer including a hollow filler is formed on a substrate and a thermal adhesive layer is formed thereon. However, this is a combination of the thermal adhesive layer which does not have adhesiveness before heated and the undercoat layer, and which is different from the constitution of the present invention. Further, the undercoat layer and the thermal adhesive layer are mixed with each other after heated, and which is different from the embodiment of the present invention. In addition, the undercoat layer reduces an energy for melting the thermal adhesive layer, and which is different from the effect of the present invention.
Japanese published unexamined applications Nos. 8-100156 and 2006-143948 disclose using a tackifier having a high softening point in an adhesive to prevent deterioration of the sensitivity of a thermal recording layer after stored for long periods. However, whether the tackifier prevents deterioration of the recording sensitivity depends on a (meth)acrylic acid ester monomer used in the adhesive.
Japanese published unexamined application No. 2004-279853 discloses a method of using a soap-free adhesive in thermal recording adhesive labels to prevent deterioration of the coloring sensitivity. This has an effect, but the recording sensitivity deteriorates under an environment of high temperature and high humidity when the (meth)acrylic acid ester monomer has an alkyl group having 4 or less carbon atoms.
Because of these reasons, a need exists for a thermal recording adhesive label having good coloring sensitivity after stored and sufficient adhesiveness with a little adhesive.